Effect Of Secondary Electron Emission Research Articles

On the in-situ determination of the effective secondary electron emission coefficient in low pressure capacitively coupled radio frequency discharges based on the electrical asymmetry effect

Abstract We present a method for the in-situ determination of the effective secondary electron emission coefficient (SEEC, γ) in a capacitively coupled plasma (CCP) source based on the γ-dependence of the DC self-bias voltage that develops over the plasma due to the Electrical Asymmetry Effect (EAE). The EAE is established via the simultaneous application of two consecutive radio-frequency harmonics (with a varied phase angle) for the excitation of the discharge. Following the measurement of the DC self-bias voltage experimentally, Particle-in-Cell/Monte Carlo Collision (PIC/MCC) simulations coupled with a diffusion-reaction-radiation (DRR) code to compute the argon atomic excited level dynamics are conducted with a sequence of SEEC values. The actual γ for the given discharge operating conditions is found by searching for the best match between the experimental and computed values of the DC self-bias voltage. The γ ≈ 0.07 values obtained this way are in agreement with typical literature data for the working gas of argon and the electrode material of stainless steel in the CCP source. The method can be applied for a wider range of conditions, as well as for different electrode materials and gases to reveal the effective SEEC for various physical settings and discharge operating conditions.

Discharge characteristics of the planar microscale gap electrodes with various geometry structures in the atmosphere environment

The discharge of microscale gaps for MEMS systems is a serious issue. To reveal the dark discharge characteristics of the microscale gap, electrodes with various gap feature sizes were prepared, and the DC current–voltage characteristics for the electrodes were implemented in atmospheric air. The results showed minor differences in dark discharge current on a nanoampere scale among electrodes with varying gap sizes of micro-protrusions. The current formation mechanism was consistent with the electron avalanche theory. Morphological characterization identified the presence of non-ideal thin-layer trailings at the electrode edges. To evaluate the effect of trailings on the dark discharge characteristics of the microscale gap structures, the electrodes with varying trailing lengths were fabricated. It showed that the current increased exponentially in the presence of long thin-layer trailing. In this case, the breakdown is most pronounced with an effective gap distance of 30 μm and a trailing length of 14.34 μm for the electrodes. For this structure, the dark discharge current was 198 nA at 36 V bias voltage, while the discharge current increased to 50 µA when the bias was increased to 37 V, and stabilized at 100 µA when the bias voltage was higher than 39 V. The rapid increase in current caused by trailing originates from the field emission as judged by the Fowler-Nordheim theory. Simulation results showed that the field strength can reach up to 3.76 × 109 V/m at a thin layer trailing length of 3.5 μm and a bias voltage of 200 V, which matches the conditions for the field emission. By calculating the effective secondary electron emission coefficient, it was found that thin-layer trailing significantly intensifies the ion-enhanced effect, further lowering the threshold for field emission and resulting in a rapid increase in microscale gap current at lower voltages.

Simulation of dust grain charging under hypervelocity impact plasma environment

Dust plasma readily forms during hypervelocity impact, which serves as a source of plasma macroscopic charge separation and strong electromagnetic fields. In this study, we examine the dynamic evolution of surface charging of aluminum dust grains with micrometer or submicrometer sizes in a hypervelocity impact plasma environment based on the theory of orbital motion limited. As dust grains traverse the expanding plasma, plasma density and temperature decrease with increasing distance from the impact point. This leads to longer relaxation times for charging equilibrium (ranging from picoseconds to microseconds) and reduced equilibrium charges. The model incorporates thermionic and secondary electron emission effects on dust grain charging processes while also examining the impacts of five heating and cooling mechanisms on the thermal equilibrium temperatures of dust grains. Near the impact point, thermal equilibrium temperatures exceed aluminum's boiling point, which results in phase transition ablation processes. As dust grain temperatures increase, thermionic emission currents may dominate charging dynamics and influence final equilibrium charge numbers. High-temperature dust grains tend to acquire positive charges. Moreover, we observe that the radius of dust grains considerably affects charging processes, and smaller grain radii correspond to low equilibrium charges and longer relaxation times.

Investigation of conditions necessary for inception of positive corona in air based on differential formulation of photoionization

Sharp point electrodes generate significant electric field enhancements where electron impact ionization leads to the formation of electron avalanches that are seeded by photoionization. Photoionization of molecular oxygen due to extreme ultraviolet emissions from molecular nitrogen is a fundamental process in the inception of a positive corona in air. In a positive corona system, the avalanche of electrons in the bulk of the discharge volume is initiated by a specific distribution of photoionization far away from the region of maximum electron density near the electrode where these photons are emitted. Here, we present a new approach to finding the inception conditions for a positive corona, which is based on a differential formulation of the photoionization problem. The proposed iterative solution considers the same inception problem that has been solved in the existing literature by using either an integral approach to photoionization or a differential formulation of photoionization and considering the inception problem as a boundary-value eigenvalue problem. The results are validated by comparisons with previous integral formulations and time dynamic plasma fluid solutions in planar and spherical geometries. The results illustrate ideas advanced in Kaptzov (1950 Elektricheskiye Yavleniya v Gazakh i Vacuume p 610) providing a physically transparent connection between an effective secondary electron emission coefficient due to volume photoionization in a positive corona system and the secondary electron emission in conventional Townsend discharge theory. The results also demonstrate the significance of boundary conditions for accurate corona solutions that are based on a differential formulation of photoionization.

Secondary electron emission and collisional effects in a two‐electron temperature plasma sheath

AbstractThe effects of secondary electron emission from the surface of a material that is in direct contact with a plasma containing two different electron groups have been investigated based on the hydro‐dynamic sheath model. The two‐electron groups present in the system have distinct densities and temperatures. They are described by the non‐extensive distribution function, whereas cold ions are assumed to be fluid elements. In addition, the effect of ion‐neutral collision has also been taken into account. The study reveals that the electron non‐extensivity has a significant effect on the secondary electron population as well as space charge deposition near the wall. The results obtained from the study will provide a better understanding of the plasma‐wall transition region under an electron‐emitting condition.

Application of optical emission spectroscopy to predict the composition of films during reactive magnetron sputtering of Ti-Al composite targets

The processes of reactive magnetron sputtering of Ti-Al composite targets with different Al/Ti ratios have been studied. The dependences of the deposition rate, discharge voltage, elemental composition, and intensity of the control lines of optical plasma radiation on the oxygen concentration in the Ar/O2 gas mixture are established. It is shown that during reactive sputtering of Ti-Al composite targets, the discharge voltage is determined by the effective ion-induced secondary electron emission coefficient (SEEC) which depends on the area occupied by metals on the target, the degree of their oxidation, and the SEEC of these metals and their oxides. The rate of the deposition of TixAl1-xOy films in the metallic and transition sputtering modes increases proportional to the concentration of Al in the target, but the relative concentration of metals in the deposited films depends on the oxygen concentration in the Ar/O2 gas mixture and is determined by the reactivity of the target materials. It has been shown by optical emission spectroscopy (OES) that the ratio of the atomic concentrations of Al and Ti in deposited TixAl1-xOy films unambiguously depends on the ratio of the intensity of the control optical lines of aluminum AlI and titanium TiI in plasma. This makes it possible to use the OES method to predict the content of metals in a film during reactive magnetron sputtering of Ti-Al targets.

Standing wave instability in large area capacitive discharges operated within or near the gamma mode

Large-area capacitive discharges used for plasma deposition operate in a regime where both electromagnetic and secondary electron emission effects are important. The standing wave shortened wavelength in the presence of plasma depends on the sheath size, and in the γ mode, the secondary electron multiplication controls the sheath physics. Near the α-to-γ transition, and within the γ mode, the sheath width typically varies inversely with the discharge voltage, and large center-to-edge voltage (standing wave) ratios may exist. This can give rise to a standing wave instability, in which the central voltage of the discharge grows uncontrollably, for a given voltage excitation at the discharge edge. Using a simple model, we determine the discharge equilibrium properties, the linearized stability condition, and the nonlinear time evolution. For sufficiently large areas, we show that a discharge equilibrium no longer exists above a critical edge voltage at marginal stability.

Experiments on the Electrostatic Transport of Charged Anorthite Particles under Electron Beam Irradiation

To reveal the effect of secondary electron emission on the charging properties of a surface covered by micron-sized insulating dust particles and the migration characteristics of these particles, for the first time we used a laser Doppler method to measure the diameters and velocities of micron-sized anorthite particles under electron beam irradiation with an incident energy of 350 eV. Here, anorthite particles are being treated as a proxy for lunar regolith. We experimentally confirm that the vertical transport of anorthite particles is always dominant, although horizontal transport occurs. In our experiments, some anorthite particles were observed to have large vertical velocities up to 9.74 m s−1 at the measurement point. The upper boundary of the vertical velocities V z of these high-speed anorthite particles are well constrained by its diameter D, that is, linearly depends on D −2. These velocity–diameter data provide strong constraints on the dust charging and transportation mechanisms. The shared charge model could not explain the observed velocity–diameter data. Both the isolated charge model and patched charge model appear to require a large dust charging potential of −350 to −78 V to reproduce the observed data. The microstructures of the dusty surface may play an important role in producing this charging potential and in understanding the pulse migration phenomenon observed in our experiment. The presented results and analysis in this paper are helpful for understanding the dust charging and electrostatic transport mechanisms in airless celestial bodies such as the Moon and asteroids in various plasma conditions.

Conventional and non-conventional diagnostics of a stable atmospheric pressure DC normal glow microplasma discharge intended for in situ TEM studies

A simple setup utilizing parallel flat electrodes with a 50–150 μm interelectrode distance divided by a Kapton spacer with a 1 mm diameter whole as discharge region intended for in situ transmission electron microscope studies is presented. The rather small setup operated in Ar or He results in an atmospheric pressure DC normal glow discharge and is investigated using various diagnostics. I–V characteristics show a glow-like behavior of the microplasma. Significant differences due to the working gas, electrode material and electrode distance have been observed. Currents in the range of 0.5–3 mA resulted in electrode potentials of 140–190 V for most experimental conditions. Optical emission spectroscopy and imaging revealed stable plasma operation and enabled the determination of current densities (approx. 16 mA mm−2 for He, or 28 mA mm−2 for Ar) independent of the input current as the discharge channel grows in diameter. Sheath thicknesses in the range of a few μm have been calculated by the collision-dominated Child–Langmuir law and trends are confirmed by the optical imaging. Energy flux measurements revealed a pronounced effect of ions on the measurement process and resulted in high energy fluxes locally up to 275 W cm−2. Effective secondary electron emission coefficients ranging from 1 to 1.6 depending on the discharge conditions have been determined based on the energy balance at the cathode.

Ar/SF6 plasma simulation for dual-frequency capacitively coupled plasma incorporating gas flow simulation and secondary electron emission

We developed the coupled calculation of plasma and gas flows in simulations for dual-frequency excited Ar/SF6 plasma. By focusing on the effect of secondary electron emission (SEE), we varied SEE coefficient γ and determined γ = 0.04 from the comparison of calculation results with the experimental results. The dependence of electron density on spatial distribution and SF6 gas partial pressure was compared between calculation and experimental results. As a result, at SF6 = 5.0 sccm, the calculated electron densities at the center and edge were almost the same as the experimental results. Furthermore, at SF6 = 2.5 sccm, the error from the experiment including the spatial distribution was in the range of −11.03 to 4.11%, and the results of coupled calculation of plasma and gas flows in simulations can reproduce the experimental results under at a SF6 partial pressure in the range from 2.5 to 5.0 sccm.

Numerical study of the effect of secondary electron emission on the sheath characteristics in q‐non‐extensive plasma

AbstractIn this paper, a plasma sheath containing primary electrons, cold positive ions, and secondary electrons is studied using a one‐dimensional fluid model in which the primary electrons are described by q‐non‐extensive distribution according to the Tsallis statistics. Based on the Sagdeev potential method and the current balance relation, a modified sheath criterion, and floating potential are established theoretically. The effect of secondary electron emission on q‐non‐extensive plasma sheath characteristics have been numerically examined. A significant change is observed in the quantities characterizing the non‐extensive plasma sheath with the presence of the secondary electrons. It is found that the sheath properties with super‐extensive distribution and sub‐extensive distribution are different compared with plasma sheath with Maxwell distribution .

Effect of secondary electrons on SGEMP response

It is difficult to effectively shield the system generated electromagnetic pulse (SGEMP), which can significantly affect the performance of important electronic devices and infrastructure, such as low-orbit spacecraft. Numerical simulation is an essential way to study the SGEMP response. However, many previous studies ignored or simplified the effect of secondary electron emission in their models. In this paper, a three-dimensional electromagnetic particle-in-cell numerical simulation model is developed to evaluate the effect of secondary electrons on the SGEMP response of two typical structures (external SGEMP and cavity SGEMP, respectively) under different current densities (0.1–100 A/cm<sup>2</sup>) and different materials (Al, Cu and Au). A right cylinder or cylindrical cavity with a length of 100 mm is used. The photoelectrons produced by the interaction between the X-ray photon and metal are emitted from one end of the system and assumed to be monoenergetic. The photoelectron pulse follows a sine-squared distribution, and its full width at half maximum is 1 ns. Some important parameters of secondary electrons are discussed and summarized, including the emission coefficients of elastically and inelastically backscattered electrons, as well as the probability density functions of emission angles and energies. The results show that ignoring the secondary emission in the simulation model leads the peak electric field to be underestimated by twice-thrice, and the duration of electric field response by more than 10%. The oscillation frequency and the amplitude of the second peak of the tangential magnetic field are also increased, with the secondary electrons considered. Among various types of secondary electrons, backscattered electrons have a dominant effect on the change of SGEMP. The effect of true secondary electrons is about 1/5 of that of backscattered electrons. The effect of secondary electrons on SGEMP response increases with a higher atomic number of the material used in the system, mainly due to higher backscattering emission coefficient and a high ratio of high energy inelastically backscattered electrons. The secondary electrons will influence the response of the external SGEMP only when the space charge effect is strong (high X-ray fluence). While the response of the cavity SGEMP is more easily affected by the secondary electrons even at a relatively low X-ray fluence. This paper helps to better obtain the SGEMP response of a specific device under strong radiation through numerical simulation.

Numerical investigation of secondary electron emission effect on the dusty plasma sheath with superextensive electrons

We have investigated the structure of a magnetized sheath of dusty plasma in the presence of secondary electrons emitted by the micro-size dust particles in the context of the Tsallis statistics. The fluid model is used to analyze numerically the effects of the nonextensivity parameter q on the emission of secondary electrons and therefore, on the sheath structure as well as the dust dynamics. The results show that the secondary emission yield increases with the decrease of the parameter of nonextensivity q and consequently, the dust charge becomes less negative with its range of values playing a primordial role in the secondary electron emission rate. The quantities characterizing the sheath are significantly affected by the secondary electron emission (SEE) from the dust. It is seen that as the SEE rises at a given value of q(q<0.91), the sheath potential decreases as well as its absolute value at the wall. In addition, the dynamics of the dust particles is also affected by the emission of secondary electrons.

Effect of secondary electron emission on the guiding properties of insulating capillaries

SynopsisThe transmission of 1 keV Ar+ ions through a macroscopic glass capillary is simulated by taking explicitly into account the secondary electron (SE) emission generated by ion impacts inside the capillary. The SE emission re-distributes the injected charge in the capillary wall and thus modifies the guiding power of insulating capillaries. Especially for high beam intensities, SE are found to be crucial for explaining the observed time-evolution of the transmitted intensities.

Study of Dust Acoustic Wave Propagation in a Lorentzian Dusty Plasma in Presence of Secondary Electron Emission

In this paper, we have investigated the effect of secondary electron emission on the propagation characteristics of linear dust acoustic waves in dusty plasmas consisting of suprathermal inertialess primary electrons, secondary electrons, Boltzmann distributed ions, and negatively charged inertial dust grains. The suprathermal electrons are assumed to follow kappa velocity distribution. We have derived the linear dispersion relation and shown that real and imaginary frequencies of the waves are influenced by the strength of secondary electron emission as well as suprathermal electron population.

The effects of electron surface interactions in geometrically symmetric capacitive RF plasmas in the presence of different electrode surface materials

Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are performed to investigate the asymmetric secondary electron emission (SEE) effects when electrons strike two different material electrodes in low pressure capacitively coupled plasmas (CCPs). To describe the electron-surface interactions, a realistic model, considering the primary electron impact energy and angle, as well as the corresponding surface property-dependent secondary electron yields, is employed in PIC/MCC simulations. In this model, three kinds of electrons emitted from the surface are considered: (i) elastically reflected electrons, (ii) inelastically backscattered electrons, and (iii) electron induced secondary electrons (SEs, i.e., δ-electrons). Here, we examined the effects of electron-surface interactions on the ionization dynamics and plasma characteristics of an argon discharge. The discharge is driven by a voltage source of 13.56 MHz with amplitudes in the range of 200–2000 V. The grounded electrode material is copper (Cu) for all cases, while the powered electrode material is either Cu or silicon dioxide (SiO2). The simulations reveal that the electron impact-induced SEE is an essential process at low pressures, especially at high voltages. Different electrode materials result in an asymmetric response of SEE. Depending on the instantaneous local sheath potential and the phase of the SEE, these SEs either are reflected by the opposite sheath or strike the electrode surface, where they can induce δ-electrons upon their residual energies. It is shown that highly energetic δ-electrons contribute significantly to the ionization rate and a self-bias forms when the powered electrode material is assumed to be made of SiO2. Complex dynamics is observed due to the multiple electron-surface interaction processes and asymmetric yields of SEs in CCPs.

Influence of the Thermo-Field Electron Emission from the Cathode with a Thin Insulating Film on the Film Emission Efficiency and Ignition Voltage of the Townsend Gas Discharge

A model of the thermo-field electron emission from the metal cathode with a thin insulating surface film at temperatures of 200–400 K is developed. An expression for the film emission efficiency in the gas discharge is obtained. The efficiency is equal to the fraction of electrons emitted into the film from the metal substrate, which enter the discharge volume and increase the effective secondary-electron emission yield of the cathode. It is shown that the thermo-field mechanism of electron emission influences noticeably the ignition voltage of the low-current discharge with such cathode at rather low temperatures exceeding the room temperature by less than 100 K.

Modeling of an Impact of Thin Insulating Film on the Electrode Surface on Discharge Ignition in Mercury Illuminating Lamps at Low Ambient Temperatures

The mixture of argon and mercury vapor with temperature-dependent composition is used as the background gas in different types of gas discharge illuminating lamps. The aim of this work was to develop a model of the low-current discharge in an argon-mercury mixture at presence of a thin insulating film on the cathode and to investigate the influence of film on the discharge ignition voltage at low ambient temperatures. When discharge modeling, we used the obtained earlier expression which describes dependence of the mixture ionization coefficient on temperature. When there was a thin insulating film on the cathode the model took into account that positive charges are accumulated on its surface during the discharge. They generate an electric field in the film sufficient for the field emission of electrons from the metal substrate of the electrode into the insulator and some of them can overcome the potential barrier at the film outer boundary and go out in the discharge volume improving emission characteristics of the cathode.Calculations showed that at a temperature decrease the electric field strengthes in the discharge gap and the voltage in it are increased due to reduction of the saturated mercury vapor density in the mixture followed by the decrease of its ionization coefficient. Existence of a thin insulating film on the cathode surface results in an increase of the cathode effective secondary electron emission yield which compensates the reduction of the mixture ionization coefficient value.The results of discharge characteristics modeling demonstrate that in case of the cathode with an insulating film the discharge ignition becomes possible at a lower inter-electrode voltage. This ensures outdoor mercury lamp turning on at a reduced supply voltage and increases its reliability under low ambient temperatures.

Modeling of the Influence of the Thickness of an Insulating Film on a Cathode Surface on its Effective Secondary-Electron Emission Yield in Low-Current Gas Discharge

A model of low-current (Townsend) gas discharge in the presence of a thin insulating film on the surface of a cathode is formulated. It takes into account, along with ion-induced secondary-electron emission from the cathode, also the field emission of electrons from the cathode metal substrate into the film under a strong electric field, which is generated in the insulator when the current flows in the discharge. The emission efficiency of the film and the discharge characteristics are calculated as functions of its thickness. It is shown that the experimentally observed nonmonotonic dependences of the effective secondary-electron emission yield of the cathode and discharge ignition voltage on the film thickness can be explained by the nonuniformity of the electric-field distribution across the film.

Effects of secondary electron emission on plasma characteristics in dual-frequency atmospheric pressure helium discharge by fluid modeling**Project supported by the National Natural Science Foundation of China (Grant No. 11505089).

A one-dimensional (1D) fluid simulation of dual frequency discharge in helium gas at atmospheric pressure is carried out to investigate the role of the secondary electron emission on the surfaces of the electrodes. In the simulation, electrons, ions of He+ and , metastable atoms of He* and metastable molecules of are included. It is found that the secondary electron emission coefficient significantly influences plasma density and electric field as well as electron heating mechanisms and ionization rate. The particle densities increase with increasing SEE coefficient from 0 to 0.3 as well as the sheathʼs electric field and electron source. Moreover, the SEE coefficient also influences the electron heating mechanism and electron power dissipation in the plasma and both of them increase with increasing SEE coefficient within the range from 0 to 0.3 as a result of increasing of electron density.